Processing-structure-property investigation of blown HDPE films containing both machine and transverse direction oriented lamellar stacks

Abstract

A series of blown films were prepared using two high density polyethylene resins of differing molecular weight and molecular weight distribution, using a high stalk process. Both the resins were processed at three frost line heights (FLHs) and three draw down ratios to determine the effect of processing parameters and resin characteristics on final film morphology and mechanical properties. By changing the FLH and the time to initiate transverse direction (TD) expansion, the relative number of lamellae stacked both perpendicular and parallel to the machine direction (MD) could be controlled for a constant blow up ratio (BUR) of 4:1. It was determined that the proportion of lamellae stacked parallel to the MD increased with increasing FLH. This effect was found to be related to the relaxation behavior of the melt and bubble shape. Increasing the stress in the stalk region was observed to lead to a reduction in stalk diameter just prior to bubble expansion, resulting in a greater effective BUR. Film morphology was observed to strongly influence end mechanical properties. Elmendorf tear resistance was found to increase in the MD and decreased in the TD with increasing FLH. The dart impact strength of these films was characterized with the surprising result that for one of the two resins studied, the dart impact increased with decreasing film gauge.